TRANSMEMBRANE RESIDUES OF THE PARATHYROID-HORMONE (PTH) PTH-RELATED PEPTIDE RECEPTOR THAT SPECIFICALLY AFFECT BINDING AND SIGNALING BY AGONIST LIGANDS/

Polar residues within the transmembrane domains (TMs) of G protein-cou pled receptors have been implicated to be important determinants of re ceptor function. We have identified mutations at two polar sites in th e TM regions of the rat parathyroid hormone (PTH)/PTH-related peptide receptor, Arg-233 in TM 2 and Gln-451 in TM 7, that caused 17-200-fold reductions in the binding affinity of the agonist peptide PTH-(1-34) without affecting the binding affinity of the antagonist/partial agoni st PTH-(3-34). When mutations at the TM 2 and TM 7 sites were combined , binding affinity for PTH-(1-34) was restored to nearly that of the w ild type receptor. The double mutant receptors, however, were complete ly defective in signaling cAMP or inositol phosphate production in res ponse to PTH-(1-34) agonist ligand. The results demonstrate that Arg-2 33 and Gln-451 have important roles in determining agonist binding aff inity and transmembrane signaling. Furthermore, the finding that resid ues in TM 2 and TM 7 are functionally linked suggests that the TM doma in topology of the PTH/PTH-related peptide receptor may resemble that of receptors in the rhodopsin/beta-adrenergic receptor family, for whi ch structural and mutagenesis data suggest interactions between TMs 2 and 7.